In recent wireless communication, adaptive control according to propagation environment has been performed. For example, in LTE (Long Term Evolution) system of 3GPP (3rd Generation Partnership Project), adaptive control of a CQI (Channel Quality Indicator), a PMI (Precoding Matrix Indicator), and an RI (Rank Indicator) has been known. The CQI is a value used to decide a modulation scheme and a coding rate, and the PMI is a value used for a transmission side to apply a weight in order to increase a signal gain at a reception side. The RI is a value corresponding to the number of transmission streams of a radio signal. In the LTE system, spectral efficiency is improved by adaptively controlling the CQI, the PMI, and the RI.
For example, a wireless device such as a portable telephone which is a receiving station of a radio signal receives a radio signal transmitted from a transmitting station through a receiving antenna, and estimates a propagation path value and noise power of each sub carrier based on a known signal such as a pilot signal. First, regarding a rank 1, the wireless device calculates a metric (for example, channel capacity) on all precoding matrices using a propagation path value and noise power for each sub carrier. Then, the wireless device extracts a largest metric of each sub carrier, and adds largest metrics of all sub carriers. The wireless device similarly performs this process on each of ranks 2 to 4. The wireless device determines a rank, a PMI, and a CQI that lead to a largest metric based on a calculation result in each rank, and feeds the determination result back to the transmitting station.
A wireless device which is the transmitting station encodes transmission data based on the fed-back CQI using an ECC (error correction code) such as a turbo code, and executes digital modulation QPSK (Quadrature Phase Shift Keying) or QAM (Quadrature Amplitude Modulation) on the encoded data. Here, in the LTE system, a coding rate and a modulation scheme of error correction are controlled for each codeword. The codeword is mapped to a layer, and a transmission data vector of an appropriate size is generated based on the fed-back RI. The transmission data vector generated in this way is subjected to precoding based on the fed-back PMI and then transmitted to the wireless device functioning as the receiving station.
Reference may be had to, for example, Japanese National Publication of International Patent Application No. 2009-514460.
However, in the related art, consideration is not given on how to reduce a computation cost in PMI estimation.
In other words, in the related art, a metric is calculated on all ranks and PMIs, and an optimal rank and PMI are selected. Meanwhile, in the 3GPP, the LTE-A (Advanced) that implements communication at a speed higher than the LTE has been discussed. Unlike the LTE, in the LTE-A, a precoding matrix W is represented by a combination of two types of matrices W1 and W2, for example, W=W1W2. That is, in the related art, consideration has not been given on a detailed PMI/RI estimating method when two precoding matrices of W1 and W2 are combined. Compared to the LTE, the number of PMIs of the LTE-A increases to be 16 times in ranks 1 and 2, 4 times in a rank 3, and twice in a rank 4, and ranks 5 to 8 are newly added. Thus, since the number of PMIs and the number of ranks to be searched increase, when a technique of performing a full search as in the related art is applied, the computation cost enormously increases.